With master cylinders there is the problem that the lost travel to be overcome when actuating the master cylinder is relatively significant, i.e. the driver of a motor vehicle has to press down the brake pedal to a relatively high degree before the pressure in the brake system is increased, thus braking the vehicle. The reason for this relatively long lost travel resulting from actuating the brake is primarily that pressure pistons in the master cylinder, starting from an idle position of the pressure pistons at the beginning of the actuation, must be displaced by a relatively great extent in the direction of the actuation before the pressure fluid connection between the pressure fluid reservoir and the pressure chambers in the master cylinder is interrupted. No significant amount of pressure can be built up in the pressure chambers of the master cylinder unless this pressure fluid connection is interrupted.
Principally, however, a smallest possible lost travel is required because this guarantees a quick reaction of the brake system giving the driver the sensation that his brake system is properly working.
The brake system is first filled with brake fluid at the manufacturer's site by means of vacuum filling. This means that the system is evacuated before being filled with pressure fluid, i.e. the air is completely removed from the brake system. After having been evacuated, the brake system is filled with pressure fluid under pressure. Due to the evacuation there is no need for a complex, retroactive venting of the system.
A filling head is mounted on the pressure fluid reservoir, which serves for the evacuation and the vacuum filling. During evacuation it is necessary to maintain the closing element of the valve in its opening position so that the air can escape unhindered from the brake system through the reservoir.
U.S. Pat. No. 6,438,955 B1 discloses a master cylinder with a valve arrangement for reducing the lost travel, in which the valve arrangement includes a closing element arranged in a reservoir bore, the valve seat thereof being built by a reservoir connecting socket and, due to a pressure difference prevailing between the pressure chamber and the pressure fluid reservoir, being movable into an opening position or a closing position. In the closing position there is a throttled pressure fluid connection between the pressure chamber and the pressure fluid reservoir.
A disadvantage of this valve arrangement is that the closing element is arranged directly at the pressure fluid reservoir, which may cause significant tolerances having a negative effect on the reduction of the lost travel.
It is, however, particularly unfavorable that the closing element is not in its opening position due to the pressure difference caused during the evacuation of the brake system, but is moved into the closing position thus hindering the air to escape from the brake system.